If want to check focus, you need to take a few extra steps. Just turning on the LCD and glancing at it won’t cut it. The small screen makes everything look more in focus because it can’t show you all the pixels at once. The reduced image size creates the illusion of a sharper image.
If you want to really see what is in focus, you’ll need to zoom, and then zoom some more. If you’re using a zoom lens, zoom in as tight as possible on your subject. Zooming in on an area like the eyes works well; a button on a shirt works well too. You’ll then need to digitally zoom.
Typically, you’ll find a Zoom button (look for a magnifying glass with a plus symbol in it) on your camera. Pressing it will enlarge the image on your screen and only show you part of the image. You may need to use the command dial to navigate around the zoomed in pixels. Find the detail area that you want to focus on.
Zooming in on your LCD can help you check focus before you roll a video shot.
You can then use the focus ring on your camera to tweak the focus. Make minor turns to find the ideal focus. If needed, adjust the aperture and ISO settings of your camera to refine the depth of field. When you’re satisfied, you can either press the Zoom Out button or just press the Record button to roll the camera.
Be sure to check out the book — Creating DSLR Video: From Snapshots to Great Shots